What is it that commands us, in good writing? There are a few fiction writers whose words reach out to us from the very first sentences of a book, compelling our assent and our delight. Toíbín is one of these writers, for me. Taste – the leap of response to a particular writer – feels sensuous rather than intellectual. Isn't it just there? The eyes – or the ears, if we're listening to something read aloud – seem crucially involved in conveying that inward clinch of critical conviction, just as in attending to paintings or music. Reading down the first page of Toíbín's new novel, Nora Webster, I know that this novel is the real thing, rare and tremendous. But how do I know?

The setting is Enniscorthy, in Ireland, in the late 1960s, and Nora Webster's husband, Maurice, has recently died. When the novel opens, Nora's next-door neighbour stops her on her way inside the house, commiserating with her over all the well-wishers calling in. "You must be fed up of them," he says. "Will they never stop coming?" Even as he commiserates, he is clearly part of the problem, waiting for the response she doesn't want to supply, pressuring her to give away how she is feeling. "They mean well. People mean well," Nora says. Her neighbour was "using a new tone with her, a tone he would never have tried before. He was speaking as though he had some authority over her". Fending off his interest, Nora "has to bite her lip to keep the tears back". We feel her danger; without her husband's strong presence sealing her apart inside the world of her marriage and her family, she is newly exposed to the probing and intrusions of other people. But we feel her resistance, too, her effort to hold off this unwanted interest, hold herself apart, keep something hidden and protected that must not be betrayed.

Like Nora, we feel tone on the skin and in the body; it produces a visceral response. Toíbín's style is distinctive, though it's the opposite of what is usually called "style" – there is no exhibition of cleverness, or highly ornamented manner, or any figurative or strenuously descriptive language. "When they came back it was dark. Nora had slept for a while and was now in the sitting room, having found a two-bar electric fire and turned it on." Or, "Nora sipped the tea and put down the cup and rested her head against the back of the chair." Nothing could be plainer than this prose, whose plainness is familiar from the rest of Toíbín's writing. Someone said to me once – not uncritically – that reading Toíbín was like drinking a glass of water. (The plainness makes his attraction to Henry James's extravagance all the more interesting.)

The whole novel is done like this, step by chronological step, from inside Nora's consciousness, following where she goes, knowing what she knows and nothing else. Toíbín restricts himself to the language and syntax his character might have used, subtly flavoured with the rhythms and idiom of this community. ("You should see Peggy Gibney," Nora says to her sister. "She's grander than your friend Dilly. Almost too grand to move.") It's written in strict chronological sequence, as time carries her forwards from the rawly traumatic loss of her beloved husband – a schoolteacher who has died young, of heart disease. She visits her sisters and aunt, goes back to the office work she thought she had left behind forever when she married, makes the decision to sell off the seaside house where the family had been happy. She watches her four children deal with their grief, each in their own way, mostly without trying to intervene or speak to them about it.

In the background to these all-absorbing private lives, the troubles begin to impinge from north of the border. Nora's husband had a history of involvement with Fianna Fáil politics, and now her daughter is taking part in protests in Dublin. At first Nora wonders what Maurice would have made of these new developments; in marriage, their thoughts and judgments had grown closely entwined. As time passes, she makes less reference to him and we are privy to her thoughts shifting almost imperceptibly into their own new shapes, leaving his influence behind. She buys a stereo record player and begins listening to the classical music her husband has never cared for – he was alert to the affectations of the classical crowd, suspicious of their pretensions to superiority. But Nora starts making friends in a new set, and taking singing lessons. The depths of feeling in the music – the cello in Beethoven's Archduke Trio becomes a kind of touchstone for her – answer to her grief, and also awaken elements in her sensibility which haven't had expression before. She imagines a different life she might have had, involved in making music.

The plainness of this prose has the same difficult scrupulousness as the most baroque good style; both are poles apart from the slapdash notation of a lazy shorthand. In less sure hands, these simplicities – one thing after another, after another – could become banal; ordinary life seems banal until it has gone, then it becomes the past and has extraordinary power to move us. This writer's withholding – of commentary, of explication, of any verdict, on the life he renders – is so striking that it's almost an inverted extravagance in itself. And the withholding is as intrinsic to the whole feel of the novel as the quality of light is intrinsic in a painting; it's also part of the novel's mimetic truth to life. There is no explication adequate to what has happened – a man is dead before his time, his family is stricken. If this experience has any meaning, it can only be opaque and deeply hidden; a writing that found its meaning easily would be suspect, fake. Yet every so often the novel's austere reserve does crack open, and to powerful effect. Nora watches the film Gaslight on television with her sons and thinks that Ingrid Bergman's performance "evoked something hidden and strange, as Maurice's absence, his body in a grave, must seem hidden and strange to the boys". Or, after singing "The Last Rose of Summer", "it came to her how they would all take their turn in the world, shadows within shadows, as Maurice had done and her mother had done … " This is richly beautiful precisely because it's so hard-won. If it's a glass of water then it's the glass of water in a Chardin still-life, momentous, and made with consummate art.

Nora Webster will remind readers of Brooklyn: it's set in a similar era, and they are both told from the woman's point of view – in fact, the mother from Brooklyn makes a brief appearance near the beginning of the new novel. Brooklyn had its inbuilt narrative tension, following Eilis to the US and then hanging on her decision over where to make her life and with which man. It must have been more difficult to find the right shape for Nora Webster, whose material is more linear and less dramatic; the drama of Maurice's death is over before anything begins, and only comes to us through Nora's haunted remembering. The story takes place in the aftermath. And yet Toíbín follows a strong line through Nora's loss and into the shape of her new life, ending with a wonderful sequence where she seems to stray into an underworld of awareness, making contact with her ghosts, her husband and dead mother.

It is clear that Toíbín has drawn closely on his own mother's story for Nora Webster: presumably he made his young self into Donal, Nora's third child, the moody incommunicative boy who stammers and sees the world through his camera. It is poignant that Donal and his mother can't easily talk and seem almost estranged – he confides in his aunt instead, who is simpler and gentler. Donal and Nora are too much alike: solitary, hidden, opening more readily to their beloved music or photography than to other people. The novel, no doubt, is the son's belated tribute of understanding, and it does everything we ought to ask of a great novel: that it respond to the fullness of our lives, be as large as life itself.